1. Field of the Invention
The present invention relates to a charging system for a satellite communication system of a demand assignment multiple access scheme and, more particularly, to a charging system for an SCPC (Single Channel Per Carrier) communication apparatus for transmitting speech and data by using subscriber terminal units in a satellite communication system of the demand assignment multiple access scheme.
2. Description of the Relevant Art
FIG. 1 is a block diagram showing a conventional satellite communication charging system.
Conventionally, in satellite communication, an SCPC communication scheme has been used, in which one narrowband carrier is assigned to one speech channel. In addition, satellite communication has been performed by the demand assignment multiple access scheme (to be referred to as the DAMA scheme hereinafter) to effectively use carriers. Referring to FIG. 3, a call connection between two terminals (subsidiary stations 2A and 3A) to be connected by this scheme is performed by one satellite link. That is, while a call connection is maintained, speech channel frequencies (f1v and f2v) are separated from control channel frequencies (f1c and f2c) which are assigned to and used by a parent station apparatus 1A to monitor and control call connections with respect to the whole network. Therefore, the two subsidiary stations 2A and 3A are released from monitoring and control of the parent station apparatus 1A so as to be isolated therefrom.
Assume that a terminal to which a call connection is performed is a device, such as a pay phone set, which requires charging. In this case, charging with respect to the call connection must be performed by the pay phone set by itself because communication between the subsidiary stations is isolated from the parent station, as describe above.
As described above, in a conventional satellite communication charging system, charging must be performed by a pay phone set by itself. For this reason, a ROM (Read Only Memory) in which charging data is stored is incorporated in each pay phone set. In charging, a pay phone set on the originating side discriminates area data on the terminating side on the basis of its telephone number, and coins are received in accordance with a unit time written in the ROM.
The following two problems to be solved, however, are posed in such a conventional satellite charging system.
First, protection against overcharges needs to be enhanced. More specifically, in the conventional scheme, no satellite link quality check result is reflected in charging with respect to the pay phone set on an originating side. Even if, therefore, an abnormality occurs in the satellite link, for some reason, to result in a speech communication failure, charging is continued unless the caller performs an ON-hook operation. This may cause an overcharge.
Second, when the manager of the network tries to change the rate system, the ROM in each pay phone set must be replaced. In general, owing to the communication network based on satellite communication, subsidiary stations to which terminal devices such as pay phone sets are connected tend to be located in a wide range of areas Therefore, difficulty is expected in replacing ROMs. As the network increases in size, it becomes more difficult to change the rate system.